The present invention relates to a drive system for electric motors, and more particularly to a drive system that allows energy recovery from a plurality of electric motors during a braking operation.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
A drive system of this type is illustrated in FIG. 1 and includes a central power supply 2 having a line-commutated converter 12 and a DC/DC converter 14, as well as a plurality of inverters 4. A DC-input of each of the inverters 4 is connected to an output of the DC/DC converter 14, whereas the respective AC outputs of the inverters 4 are connected to corresponding multi-phase electric motors 10. The input of the line-commutated converter 12 is connected via chokes 20 to terminals U, V, and W of a three-phase power line (not shown). The output of the line-commutated converter 12 is connected to the input of the DC/DC converter 14, with a first buffer capacitor 16 connected across the input of the DC/DC converter 14 and second buffer capacitor 18 connected across the output of the DC/DC converter 14. From the phase voltages UU, UV, and UW of the power line, the line-commutated converter 12 generates a DC voltage UDCN which is applied across the first buffer capacitor 16. The DC/DC converter 14 generates from the DC voltage UDCN a specified regulated DC voltage UDCV, also referred to as supply voltage, which is applied across the second buffer capacitor 18.
It may be necessary to brake inverter-powered motors 10 by converting mechanical energy into electrical energy which is then transferred to the second buffer capacitor 18. Because the braking process requires that all motors 10 switch to generator mode at the same time, no load is available to use the recovered energy. Moreover, the recovered energy cannot be returned to the power line, because a line-commutated rectifier 12 operating as a converter is arranged on the power line side the central power supply 2. Even if the line-side converter 12 were able to recover energy, the recovered energy could not be returned to the power line, because the DC/DC converter 14 on the output side of the central power supply 2 can transmit energy in only one direction. The supply voltage UDCV across the second buffer capacitor 18 must therefore be prevented from exceeding or even reaching a predetermined limit value, in which case the drive system would need to shut down for safety reasons.
FIG. 2 shows in more detail a brake circuit 22 which is intended to prevent an overvoltage across the capacitor 18 and hence also an emergency shut down of the drive system. The brake circuit 22 includes a brake controller 24, also referred to as a brake chopper, and a brake resistor 26. The brake chopper 24 is connected in parallel with the terminals 6 and 8 of the regulated DC voltage output of the central power supply 2. The brake circuit 22 maintains the supply voltage UDCV across the capacitor 18 at a predetermined voltage level and converts the energy received from the motors 10 into ohmic losses (heat) in the brake resistor 26 which needs to be sized accordingly and may also have to be cooled. In addition, the brake resistor 26 may require a considerable amount of space.
In prior art transport systems the transport paths are configured by combining separate transport modules. Each transport module includes several rollers, at least one roller drive and a corresponding inverter that powers a motor. The transport modules are compact and only need to be assembled, i.e., mechanically and electrically connected which each other, for example by connecting the DC inputs of the inverters in parallel and then with the DC terminals of the central power supply 2, which can be located centrally with respect to the transport system or with respect to a section of the transport system. A fast braking action is required when positioning the transported items with the shortest possible cycle times and optionally handing the transported items over to other transport paths, whereby the mechanical energy is returned as electrical energy to the capacitor 18 connected across the output terminals of the central power supply. Typically, the speed of all drives must be reduced simultaneously, i.e., all drives switch simultaneously to generator mode. As mentioned above, the output of the central power supply is provided with a brake circuit to prevent an emergency shutdown of the drive system of the transport system for safety reasons. The central power supply 2 can be arranged in the center of the transport system or of a section of the transport system.
This type of drive system for a transport system generates from an AC line voltage of, for example, 380 V a DC voltage UDCN of, for example, 540 V, to which an AC-component with six times the line frequency is superimposed. Because the inverters 4 for the motors 10 require a supply voltage UDCV of, for example, 48 V, the central power supply 2 includes a DC/DC converter 14 that produces from the DC voltage UDCN the desired regulated supply voltage UDCV of, for example, 48 V. This voltage is low enough so as not to require special cable selection or special cable routing.
It would therefore be desirable and advantageous to provide a drive system, which obviates prior art shortcomings and enables energy recovery from a load without the need for a brake circuit.